The present invention relates generally to centrifugal pumps; more specifically to multi-stage centrifugal pumps with magnetic-drive assemblies.
Multi-stage centrifugal pumps are often used where single-stage centrifugal pumps do not meet the output pressure or head requirements of the particular application. Most multi-stage centrifugal pumps presently use shaft seals to isolate the pumped fluid from the motor. For example, U.S. Pat. No. 5,445,494, entitled MULTI-STAGE CENTRIFUGAL PUMP WITH CANNED MAGNETIC BEARING and issued to Hanson, discloses a seal for isolating pumped fluid from the motor and magnetic bearings. Shaft seals may leak hazardous or caustic fluids, potentially endangering the safety of pump operators. Periodic inspections of seals are required to reduce unwanted emissions from pumps.
Magnetic-drive, multi-stage pumps may be used to replace entire pumps with seals to reduce unwanted emissions. However, magnetic-drive assemblies cannot be coupled readily to multi-stage centrifugal pumps, designed for traditional seals, without first solving sundry engineering problems. Axial bearing design and axial shaft adjustment are some illustrative examples of engineering problems related to converting multi-stage pumps to magnetic-drive operation.
Many pumps have two axial bearings to meet loads in opposite axial directions as shown in U.S. Pat. No. 5,269,664, entitled MAGNETICALLY COUPLED CENTRIFUGAL PUMP. A thrust collar is located toward the containment shell to absorb axial loads in one direction, while an auxiliary thrust collar is located toward the intake of the pump to absorb axial loads in an opposite direction. The thrust collar is located adjacent to the radial bearing or bushing. One side of the bushing acts as the mating face for the thrust collar. Therefore, the bushing has the dual, stressful role of an axial bearing member and a radial bearing. The bushing is flared or has an exaggerated radial thickness at the bearing interface to increase the reliability of the bearing. The bushing and the thrust collar must use compatible materials.
Some multi-stage pumps do not even have thrust bearings within the pump. Instead, thrust bearings are located in a motor attached to the pump. However, magnetic-drive centrifugal pumps cannot rely on the motor to handle axial loads on the impeller shaft because of the inherent dual shaft design of magnetic-drive pumps.
The shaft and impellers usually need to be axially adjustable for multi-stage pumps. The shaft and impellers are often axially aligned with split, friction collars. The collars are tightened on the shaft. Adjustable collars may shift in axial alignment when exposed to vibration, shock, and axial forces during normal operation of the pump. Some pumps incorporate axial thrust position monitors to measure the relative position of the thrust collar to the thrust bearing. Extreme axial displacement of the thrust bearing from its proper position may cause damage to the impellers if the impellers contact the walls of the internal pump chambers.
Therefore, a need exists for a magnetic-drive assembly which may be coupled to an existing multi-stage centrifugal pump to upgrade the existing multi-stage centrifugal pumps in the field to magnetic-drive operation, without replacing the entire pump. Furthermore, a need exists for a magnetic-drive, multi-stage centrifugal pump which minimizes requisite shaft length through an axially compact magnetic-drive assembly with a reliable, stable axial shaft adjustment mechanism.